1. Field of the Invention
The present invention relates to an interface for transferring workpieces such as semiconductor wafers, reticles and flat panel displays between a carrier for the workpieces and a process tool, and in particular to a system including a load port which allows various pod sizes and configurations to attach to and operate with a standard vertical interface on the front end of a process tool.
2. Description of Related Art
A SMIF system proposed by the Hewlett-Packard Company is disclosed in U.S. Pat. Nos. 4,532,970 and 4,534,389. The purpose of a SMIF system is to reduce particle fluxes onto semiconductor wafers during storage and transport of the wafers through the semiconductor fabrication process. This purpose is accomplished, in part, by mechanically ensuring that during storage and transport, the gaseous media (such as air or nitrogen) surrounding the wafers is essentially stationary relative to the wafers, and by ensuring that particles from the ambient environment do not enter the immediate wafer environment.
A SMIF system has three main components: (1) minimum volume, sealed pods used for storing and transporting wafers and/or wafer cassettes; (2) an input/output (I/O) minienvironment located on a semiconductor processing tool to provide a miniature clean space (upon being filled with clean air) in which exposed wafers and/or wafer cassettes may be transferred to and from the interior of the processing tool; and (3) an interface for transferring the wafers and/or wafer cassettes between the SMIF pods and the SMIF minienvironment without exposure of the wafers or cassettes to particulates. Further details of one proposed SMIF system are described in the paper entitled "SMIF: A TECHNOLOGY FOR WAFER CASSETTE TRANSFER IN VLSI MANUFACTURING," by Mihir Parikh and Ulrich Kaempf, Solid State Technology, Jul. 1984, pp. 111-115.
Systems of the above type are concerned with particle sizes which range from below 0.02 microns (.mu.m) to above 200 .mu.m. Particles with these sizes can be very damaging in semiconductor processing because of the small geometries employed in fabricating semiconductor devices. Typical advanced semiconductor processes today employ geometries which are one-half .mu.m and under. Unwanted contamination particles which have geometries measuring greater than 0.1 .mu.m substantially interfere with 1 .mu.m geometry semiconductor devices. The trend, of course, is to have smaller and smaller semiconductor processing geometries which today in research and development labs approach 0.1 .mu.m and below. In the future, geometries will become smaller and smaller and hence smaller and smaller contamination particles and molecular contaminants become of interest.
SMIF pods are in general comprised of a pod door which mates with a pod shell to provide a sealed environment in which wafers may be stored and transferred. At present, there are different configurations of SMIP pods owing in part to the different wafer sizes and tool interface orientations currently found in wafer fabs. In addition to existing 200 mm wafers, processing of 300 mm wafers has been introduced in recent years. Different pods and pod handling equipment are employed depending on whether processing is being performed on 200 mm or 300 mm wafers. Additionally, SMIF pods may either be bottom opening or front opening. In bottom opening SMIF pods, the pod door is provided horizontally at the bottom of the pod, and the wafers are supported in a cassette which is in turn supported on the pod door. In front opening pods, the pod door is located in a vertical plane, and the wafers are supported in parallel horizontal planes on shelves mounted within the pod shell. Such pods are commonly referred to as front opening unified pods, or FOUPs.
Bottom opening pods conventionally operate with horizontally oriented load port interfaces, and front opening pods conventionally operate with vertically oriented load port interfaces. Although 200 mm wafers may be stored and transported in either bottom or front opening pods, 200 mm wafers are typically used with bottom opening pods. Similarly, although 300 mm wafers may be stored and transported in either bottom or front opening pods, 300 mm wafers are now typically used with front opening pods.
Process tools typically include load ports affixed to their front end so that pods or stand-alone cassettes may be positioned adjacent the process tool in preparation for the wafers and/or wafer cassettes to be transferred into the process tool. For wafer fabs utilizing 300 mm wafers in front opening pods, a vertically oriented frame, commonly referred to as a box opener-loader tool standard interface (or "BOLTS" interface), has been developed by Semiconductor Equipment and Materials International ("SEMI"). The BOLTS interface attaches to, or is formed as part of, the front end of a process tool, and provides standard mounting points for a load port to attach to the process tool. The BOLTS interface also provides a reference position so that a fixed distance between the BOLTS interface and the center of a wafer on the load port may be defined. Thus, although equipment manufacturers make various configurations of load ports for 300 mm front opening pods, each load port may be operationally connected to a process tool as long as the load port is configured to attach to the defined standardized BOLTS interface frame.
In 300 mm front opening pods, a pod is typically loaded either manually or automatedly onto the load port so that the vertical pod door lies adjacent the vertical port door to the process tool. Thereafter, mechanisms within the load port advance the pod to the port, where the port door decouples the pod door from the pod shell and moves the pod door and port door together into the minienvironment and then off to the side, above or below the port. A wafer handling robot within the process tool may thereafter access particular wafers supported in the pod shell for transfer between the pod and the process tool. The wafer handling robot makes use of the known distance from the BOLTS interface to the wafer center to properly position the end effector of the robot with respect to a particular wafer being accessed.
Despite the potential increased capacity presented by chip manufacture from 300 mm wafers, several wafer fabs have been reluctant to switch from processing 200 mm wafers, and it is unlikely that an industry-wide conversion to 300 mm wafer processing will take place in the near future. The BOLTS interface was developed based on front opening 300 mm SMIF pods, and wafer fabs which utilize 200 mm wafers and/or bottom opening pods are not at present able to make use of the several advantages provided by the BOLTS interface.
Separate and apart from the problem presented by varying wafer sizes and pod types, several wafer fabs are not currently utilizing SMIF technology. Some of these fabs merely use bare cassettes, while other fabs use so-called "run boxes" around the cassettes to provide some degree of protection during storage and transport of the wafers. In either of these cases, when a cassette is transferred onto a load port, the wafers within the cassette are exposed to the cleanroom environment, where particulates and contaminants may deposit on the wafers. While it would be possible to provide a cleaner environment within a wafer fab or cleanroom surrounding a process tool, this approach is extremely expensive to implement for current wafer environment requirements.